Electrical impedance tomography method for reconstruction of biological tissues with continuous plane-stratification.

A novel electrical impedance tomography method is introduced for reconstruction of layered biological tissues with continuous plane-stratification. The algorithm implements the recently proposed reconstruction scheme for piecewise constant conductivity profiles, based on an improved Prony method in conjunction with Legendre polynomial expansion (LPE). It is shown that the proposed algorithm is capable of successfully reconstructing continuous conductivity profiles with moderate (WKB) slop. Features of the presented reconstruction scheme include, an inherent linearity, achieved by the linear LPE transform, a locality feature, assigning analytically to each spectral component a local electrical impedance associated with a unique location, and effective performance even in the presence of noisy measurements.

Cavity-enhanced biosensing utilizing plasmon resonance modes.

Traditionally, surface plasmon resonance (SPR) biosensors utilize absorption of light radiation incident upon noble metal films above the total internal reflection angles. Herein we extend the SPR phenomenon to incorporate cavity plasmon resonance (CPR) excitation of metallic films at incidence angles below the critical angle. While SPR occurs for TM polarized light only and requires very specific excitation conditions, which could be disadvantageous in some practical designs, CPR does not require complicated evanescent field excitation above the critical total internal reflection angle and can be implemented for both transverse electric (TE) and transverse magnetic (TM) fields even under normal incidence (TEM). These and other unique features of CPR enable a more flexible design of highly efficient and sensitive biosensing devices.

Optimized microbolometers with higher sensitivity for visible and infrared imaging.

Optimal absorption method for improving the sensitivity of bolometric detection is explored. We show that, in addition to its role in conventional conducting-film detection, the application of plasmon resonance absorption offers highly promising characteristics for efficient far-field thermal detection and imaging. These characteristics include good frequency sensitivity, intrinsic spatial (angle) selectivity without focusing lenses, wide tunability over both infrared and visible light domains, high responsivity and miniaturization capabilities. In this context, we examine the well-known surface plasmon resonance (SPR) regime, but also report on a new type of plasmon resonance excitation, the cavity plasmon resonance (CPR), which offers more flexibility over wide ranges of wavelengths, bandwidths, and device dimensions. Both CPR and SPR occur in metallic films, which are characterized by high thermal diffusivity essential for fast bolometric response.

Broadband absorption spectroscopy via excitation of lossy resonance modes in thin films.

It is shown that broadband absorption spectroscopy utilizing thin lossy film configurations can be optimally facilitated when applied to metallic and insulating materials. For metallic films, the zero-order highly lossy resonance mode, characterized by ultra wideband absorption behavior under normal incidence, can be shifted, under parallel-polarization oblique incidence, toward a narrow band light-wavelength surface plasmon resonance condition. Higher order low-loss modes, however, occur in thin insulating films, exhibiting Debye relaxation behavior, typical for many aqueous solutions and biological substances. They can be excited in a highly scalable and sensitive manner in various frequency bands, between light and radio frequencies.

Image series expansion for electrical impedance tomography.

A novel electrical impedance tomography method is introduced for reconstruction of layered biological tissues. The method utilizes a recently proposed image series expansion scheme in conjunction with the WKB approximation. This results in a locality feature, assigning analytically to each image term a local impedance associated with a unique layer, and thus leading to linear, efficient, and accurate reconstruction procedures.

Optimal dispersion relations for enhanced electromagnetic power deposition in dissipative slabs.

Broadband analysis of a prototype model associated with electromagnetic waves absorption in a lossy dielectric slab renders a closed-form theoretical prediction of an infinite number of optimal absorption paths in the complex refractive index domain. While for thin slabs (in terms of incident wavelength range) each path corresponds to a lossy Fabry-Perot-type resonance modes of order m=0,1,2,... and provides at least 50% absorption of the incident wave power even for ultrathin slabs, for optimal thick slabs, the fraction of absorbed power, asymptotically estimated via Lambert W function, increases up to 100% for consecutive continuation of the m=1 normal mode only.

Increased acoustic and electromagnetic energy deposition in a layered tissue model.

By analyzing and optimizing acoustic and electromagnetic waves absorption in a simplified layered model of hyperthermic configuration, it is shown that the commonly used attenuation metrics are not always proper means for determining the generally optimal parameters of typical thermal therapy problem with finite extent target The conditions, parameters and bounds for optimal (maximal) incident power absorption for the layered model have been found analytically and explicitly and are presented in terms of basic wave propagation characteristics, thus, also providing the necessary data for optimal synthesis of absorbing tissues and materials.

WKB quasistatic reconstruction of layered biological tissues.

A novel electrical impedance tomography method is introduced for reconstruction of layered biological tissues. The method utilizes a recently proposed image series expansion scheme in conjunction with the WKB approximation. This results in a locality feature, assigning analytically to each image term a local impedance associated with a unique layer, and thus leading to efficient and accurate reconstruction procedures.

Interaction of array of finite electrodes with layered biological tissue: effect of electrode size and configuration.

A hybrid scheme, combining image series and moment method has been utilized for the calculation of the intramuscular three-dimensional (3-D) current density (CD) distribution and potential field transcutaneously excited by an electrode array. The model permits one to study the effect of tissue electrical properties and electrode placement on the CD distribution. The isometric recruitment curve (IRC) of the muscle was used for parameter estimation and model verification, by comparison with experimentally obtained IRCs of functional electrical stimulation (FES)-activated quadriceps muscle of paraplegic subjects. Sensitivity of the calculated IRC to parameters such as tissue conductivity, electrode size, and configuration was verified. The resulting model demonstrated characteristic features that were similar to those of experimentally obtained data. The model IRCs were insensitive to the electrode size; however, the inclusion of the bone-fascia layer significantly increased the intramuscular CD and, consequently, increased the IRC slope. Of the different configurations studied, a four-electrode array proved advantageous because, in this case, the CD between the electrodes was more evenly distributed, providing better resistance to fatigue. However, due to the steeper linear portion of the IRC, this configuration suffered from a somewhat reduced controllability of the muscle.

Current distribution in skeletal muscle activated by functional electrical stimulation: image-series formulation and isometric recruitment curve.

The present work develops an analytical model that allows one to estimate the current distribution within the whole muscle and the resulting isometric recruitment curve (IRC). The quasistatic current distribution, expressed as an image series, i.e., a collection of properly weighted and shifted point-source responses, outlines an extension for more than three layers of the classical image theory in conductive plane-stratified media. Evaluation of the current distribution via the image series expansions requires substantially less computational time than the standard integral representation. The expansions use a unique recursive representation for Green's function, that is a generic characteristic of the stratification. This approach permits one to verify which of the tissue electrical properties are responsible for the current density distribution within the muscle, and how significant their combinations are. In addition, the model permits one to study the effect of different electrode placement on the shape and the magnitude of the potential distribution. A simple IRC model was used for parameter estimation and model verification by comparison with experimentally obtained isometric recruitment curves. Sensitivity of the model to different parameters such as conductivity of the tissues and activation threshold was verified. The resulting model demonstrated characteristic features that were similar to those of experimentally obtained data. The model also quantitatively confirmed the differences existing between surface (transcutaneous) and implanted (percutaneous) electrode stimulation.

Plane-wave spectrum approach for tilted waveguides.

Scattering of guided modes from an abruptly terminated waveguide is analyzed through an integral-equation formulation. First the boundary-value problem for a plane-stratified waveguide with arbitrary profile is reduced to a canonical system of surface integral equations. A Born-type iterative procedure is then employed to obtain a tractable solution of the scattering field at the termination. The specific choice of a tilted planar termination renders an explicit closed-form expression for the first Born approximation, represented by the plane-wave spectrum of the incident modal field modified by the appropriate Fresnel coefficient. Thus previous ad hoc formulations can be recovered as limiting cases of the suggested rigorous expansion scheme.